Ricon Corporation,
an American company specialising in the design and
manufacture of vehicle accessibility products, has specified
Penny & Giles
ICS100 in-cylinder sensors for its latest Mirage F9T Transit
Lift.

The Mirage F9T,
which is designed to enable passengers using wheelchairs to
access buses, stows under the floor of the vehicle leaving the
doorway
completely clear.

The previous
version of the transit lift used a series of switches and a cam
to indicate the position of the platform. However, in order to
stow the lift
within its enclosure without interference the company needed
to sense to
within a 6mm (+/- 3mm) tolerance whether the platform was correctly
aligned.

The cam that
actuated the switch travelled approximately 3mm for every 50mm
that the platform travels and adjustments were made by moving
the switch and
re-tightening a number of setscrews. As a movement of 1mm on
the switch
translated to an approximately 17mm movement on the platform,
adjustments
were extremely sensitive making the 6mm acceptable window very
difficult to
achieve.

These adjustments
were taking up to 15 minutes for an experienced fitter and
much longer for inexperienced personnel trying to follow the
procedure from
the manual!

As a result,
the engineering brief for the Mirage F9T specified a simple,
foolproof and fast way of setting the stow height of the platform.
Inherent
requirements in the system called for a device with infinite
resolution.

A spokesman for
Ricon Corporation says the Penny & Giles ICS100 hybrid
linear potentiometer was chosen because the technology was simple,
elegant,
and cost-effective. The conductive plastic film offered the
infinite
resolution, while the wire-wound construction offered robustness
to the
design.

"There are
other devices on the market that offer infinite resolution, but
they are more expensive due to the controls needed to read them.
For our
application, the ICS100 with infinite resolution and a 10-bit
A/D device
gave us sufficient accuracy."

Because the Mirage
F9T is hydraulic and has no closed-loop feedback
circuits, Ricon engineers had to find a way to overcome the inherent
problems with its slow response.

Explains the
spokesman: "The time-lapse between telling a valve or pump
motor to turn off and when it actually stops, is too long and
causes the
platform to overshoot the intended 6mm target."

Changing environmental
conditions also make the overshoot unpredictable.
Temperature changes will affect the viscosity of the oil. Wear
on the
bearing surfaces will change the friction between them. And different
standards of maintenance from customer to customer will add other
variables
to the lift's performance.

Ricon engineers
therefore employed an algorithm (U.S. Patent number
6,236,905) that examines the error of the last cycle and compensates
on the
next cycle - for every cycle from the first. It continually corrects
for
errors experienced on the last cycle to protect the unit against
environmental conditions that might change the characteristics
of the
hydro-mechanical system.

The system has
been in use since Ricon has been buying Penny & Giles
potentiometers and has proven to be very reliable.
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